Field of the Invention
This invention relates generally to radio systems and, more particularly, to radio systems for a vehicle that employ a high Q-factor narrow-band antenna that is tunable to a selected radio station.
Discussion of the Related Art
Vehicles can include AM/FM radio, TV and other wireless services having antennas that are tunable to, for example, amplitude modulated (AM) radio broadcast signals and frequency modulated (FM) radio broadcast signals and various modulations used worldwide for television signals. Different countries may identify AM, FM, TV and other wireless frequency bands, and frequency steps within those bands, differently. As an example, in the United States, AM signals are broadcast in the 540-1700 kHz frequency range, where each station in a particular area is assigned a 10 kHz channel in that range. Also, in the United States, FM signals are broadcast in the 88-108 MHz frequency range, where each station in a particular area is assigned a 200 kHz channel in that range. Vehicles also may employ other wideband broadcast frequency bands, such as Band 3 and TV bands.
Typically, radio antennas for vehicles are often low Q-factor broadband antennas that are able to receive broadcast signals over the entire, and normally wide, frequency band. The Q-factor is sometimes defined with respect to the bandwidth of operation of the radio. For example, the Q-factor is often known to RF engineers as the ratio of the operating bandwidth to the center frequency of the operational bandwidth. In this definition, a lower Q-factor device will have a larger operational bandwidth than a higher Q-factor device with the same center frequency.
Employing broadband antennas for vehicle radios has typically been necessary because information about which particular channel the radio is tuned to was not available. For example, because it is necessary to design AM and FM antennas to receive signals across the entire AM or FM frequency band, those antennas typically had a low performance because the impedance matching between the antenna and the front end receiver components needed to be designed to accommodate the entire frequency range. Further, noise received over the entire frequency band was processed by the receiver front end even though the radio was only tuned to a small bandwidth of that frequency band. This is also true for the other wireless broadcast services (e.g. television).
Since most vehicles have metallic skins and other metal structures, they are able to effectively shield or reduce noise over the desired frequency band that the antenna was exposed to (e.g. AM, FM, TV or other wireless services). However, with the popularity of composite structures in high performance vehicles, that signal shielding has become less available, and with the increasing number of electronics on those types of vehicles, the potential for interference from noise over the wider frequency band increases. Thus, antennas are exposed to higher levels of noise, often referred to as radiated emissions since the noise is radiated from other electronic modules on the vehicle.